The present invention relates to an ink jet recording apparatus which records image, literature, etc. on a recording medium through the use of an ink jet recording head, and relates to a method for driving the ink jet recording head.
There is a recording apparatus using an ink jet recording head among recording apparatuses such as printer, plotter, and so on. Among the recording heads, there is a recording head ejecting ink drops from a nozzle orifice by varying ink pressure in a pressure chamber. The recording head varies ink pressure by varying capacity of the pressure chamber with deformation of piezoelectric vibrator, for example. Because of that, ink pressure is controlled by varying wave shape of pulse signal supplied to the piezoelectric vibrator so as to obtain desired ink quantity, jetting speed and so on.
The pulse signals are, for example, a micro dot pulse for recording a micro dot and a medium dot pulse for recording a medium dot. The medium dot pulse consists of: an expanding element raising a voltage from a reference voltage to an expansion voltage with constant gradient of degree not discharging the ink drops; an expansion holding element holding the expansion voltage for very short time (about 1.0 microseconds); and an ejecting element lowering the voltage from the expansion voltage to the reference voltage, for example. When the medium dot pulse is supplied to a piezoelectric vibration having a longitudinal vibration mode, pressure in the pressure chamber is decreased by slow expansion of the pressure chamber with the supply of the expanding element. After the expansion holding element is supplied instantly, the pressure chamber is contracted drastically by supply of the ejecting element, and ink pressure in the pressure chamber rises with the contract so that the designated quantity of ink drop corresponding to the medium dot is ejected from the nozzle orifice.
Incidentally, stability of discharge of ink drops is required in this kind of recording apparatus.
However, in the related middle pulse, vibration of meniscus just after supply of driving pulse becomes large because the ejecting element is supplied for very short time after the expanding element is supplied. Potential difference of the ejecting element (voltage difference from reference voltage to expansion voltage) tends to become relatively large, and even at this point, vibration of meniscus just after the supply becomes large. Because of that, continuous supply of driving pulses makes discharge of ink drops becomes unstable, for example, volume and flight direction of ink drops becomes uneven.
The invention is carried out in consideration of such the circumference, the object is to provide an ink jet recording apparatus making discharge of ink drops stable, and to provide a method for driving an ink jet recording head incorporated in the recording apparatus.
In order to achieve the above object, according to the present invention, there is provided an ink jet recording apparatus, comprising:
a recording head, including a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and an actuator which varies a volume of the pressure chamber;
a drive signal generator, which generates a drive signal in which a plurality of drive pulses are arranged within a unit recording period, the drive pulses including at least a first drive pulse composed of:
an expanding element, which drives the actuator so as to expand the pressure chamber;
an expansion holding element, which drives the actuator such that the expanded state of the pressure chamber is held in a first time period roughly equal to a first natural vibration period which is a natural vibration period of ink stored in the pressure chamber; and
an ejecting element, which drives the actuator so as to contract the pressure chamber so that the ink in the pressure chamber is ejected from the nozzle orifice; and
a pulse supplier, which selects at least one drive pulse from the drive signal and supplies the selected drive pulse to the actuator.
In this configuration, since meniscus of the ink vibrates freely while the expansion holding element is supplied, the nozzle orifice is filled with the ink when the supply of the ejecting element is started so that the contraction of the pressure chamber is started from this state. Accordingly, ink ejection can be carded out substantially in a state which is so called xe2x80x9cextruding ejectionxe2x80x9d so that the designated quantity of ink drops can be ejected even if driving voltage of the piezoelectric vibrator is made low considering that the nozzle orifice is filled with ink Therefore, required external force applied to the pressure chamber is decreased so that quantity and flight direction of ink drops are made stable.
Preferably, a second time period in which the expanding element is supplied is roughly equal to the first natural vibration period.
In this configuration, contraction of the actuator can be driven in synchronizing with expansion speed of the pressure chamber when the expanding element is supplied. Accordingly, since the pressure chamber is expanded efficiently, needless vibration of meniscus can be suppressed as low as possible.
Preferably, a third time period in which the ejecting element is supplied is roughly equal to a second natural vibration period which is a natural vibration period of the actuator.
In this configuration, the actuator can be driven surely without needless action such as bending etc. when the ejecting element is supplied. Thus, the pressure chamber in expanded state can be contracted surely.
Preferably, the first time period is set in a range of 80 to 120 % of the first natural vibration period.
Preferably, the selected drive pulse is determined in accordance with quantity of ink ejected from the nozzle orifice.
Here, it is preferable that the drive signal includes at least the first drive pulse which drives the actuator so as to eject first amount of ink and a second drive pulse which drives the actuator so as to eject second amount of ink which is different from the first ink amount
Further, it is preferable that the second ink amount is less than the first ink amount.
It is preferable that the second drive pulse is arranged prior to the first drive pulse in the drive signal.
Alternatively, it is preferable that all the drive pulses in the drive signal are the first drive pulse.
According to the present invention, there is also provided a method of driving a recording head which includes a nozzle orifice, a pressure chamber communicated with the nozzle orifice, and an actuator associated with the pressure chamber, comprising the steps of:
driving the actuator so as to expand the pressure chamber so that meniscus of ink situated in the vicinity of the nozzle orifice is drawn toward the pressure chamber;
waiting until the drawn meniscus is returned in the vicinity of the nozzle orifice due to free vibration thereof; and
driving the actuator so as to contract the pressure chamber so that the ink is ejected from the nozzle orifice after the waiting step is performed.
Preferably, a time period in which the waiting step is performed is roughly equal to a natural vibration period of ink stored in the pressure chamber.